Sound recording system



Nov. 21, 1933.

R. R. SCOVILLE SOUND RECORDING SYSTEI Filed Feb. 16, 1933 mw ww \N MN N km mm NM w Mm 3 Mm INVE N TOR R R. SCUV/LLE By 6- H-Auvfl'.

ATTOR/VEV Patented Nov. 21, 1935 1,936,176 SOUND nsconnme SYSTEM Bay It. Scoville, Los Angeles, Calif., Electrical Research Products, Inc., N. Y., a corporation of Delaware assignor to New York,

' Application February 16, 1933. I Serial No. 657,019

sclaiins. (01. 179-1003) V This invention relates to systems for recording signal currents in which the mean recording position of the recording device may bevaried from the center of the available range of the characteristic of the medium by a carrier current modulated by the signal currents to be recorded.

I The object of the invention is to control the carrier current so that the load. carrying capacity of the recording device will vary in accordance with the variations in the amplitude of the signal currents.

A feature of the invention resides in the use of an electron discharge device as a source of local oscillations, and of another electron discharge device to control the supply of carrier current from the oscillator to the recording device. A rectified current, derived from the signal currents, varies the polarity of the control electrodeof the control device, thus causing a current to fiow in the input circuit of the control device. The resultant change in impedanceof the input circuit of the control device is reflected back into the input circuit of the oscillator and causes the amplitude of the oscillations to vary. f

In known recording systems, the recording device operates over a predetermined range of some selected characteristic of the medium, such as the photographic density of the mediurnor thewidth of the recording area. The recording device is the mean recording position is in the center of the predetermined range and is varied momentarily above and below this mean position by the variations in the signal currents.

As the recording mediums are not perfectly homogeneous, a certain amount of undesired noise is produced during the reproduction of the record by the irregularities in the medium. If the reproduced, or positive, record is at all times as dark as possible this-undesired noise will be reduced to a minimum. To produce this result, the exposure of the original or negativerecord must, at all times, be as small as possible. In the various known systems of recording, this result will be attained if the exposed area is a minimum or if the intensity or" the recording light is a minimum or if thetime or" exposure of the medium isa minimum. The recording device may be. mechanically adjusted so that the mean recording position is in the center of the available range of the desired characteristic. A biasing current is then applied to the recording device so that, in the absence of signal currentsthe mean recording position isdisplaced to the iowere'nd oi the available range, thus reducing the exposure oithe adjusted so that, in the absence of signal currents, V

' rent varying with a varying carrier output is tered and applied to the recording device to in- 1 original or negativerecord to "some small value. When signal currents are applied to the recording system, the biasing current is varied in accordance with some characteristic of the signal currents thusshiiting the mean recording position along 0, the selected characteristic, and increasing the mean value of the exposure to permit the recording device to record; the signal current without over-running the desired limits of the character istic of the medium. Or, inother words, the load e5v carrying capacityof the recording device is small when thesignal currents are small and increases in accordance with an increase in the signal currents so that the recording device will not be overloaded. I

, t, p In accordance with the present invention, the

- carrier current produced by a source of high frequency oscillations is varied in accordance with a characteristic of the amplitude variations of the 7 signal currents to be' recorded. The varying carrier current is thenrectified and filtered andapplied to-the recording device to control themean v recording position. The'instantaneous variations in the signal currents are independently applied to the recording device; through a recording so channel which is independent of thiscontrol circuit.

In a preferred embodiment of the invention, the high frequency oscillations are tapped off the input circuit of an oscillator and supplied 35 to a pair of electron discharge devices,' in pushpull arrangement. A portion of the signal currents is rectified and filtered, to obtain a"cur-' desired characteristic .of the signal currents, such as the envelope of the amplitude variations of the signal currents.

The-rectified signal current is applied to the common portion of theinput circuit of the pushpull control devices, and drives the potential of the control electrodes of the controldevices sufficiently positive to cause .a current to flow in the input circuit of the control devices, thus lowering the impedance of the input circuit of the control devices. As this impedance iseffectively in shunt with the inputcircuit of the oscillator, the efiectiveimpedance .ofthe input circuit of the oscillator is lowered, and the output of high frequency carrier supplied to the control devices is decreased to a greater amount than the arn pliiication of the control devices is increased. The 1 carrier output of the control devices thus varies in accordance with the envelope of the amplitude variations of the signal currents. This then rectified and filcrease the load carrying capacity of the recording device.

The drawing diagrammatically illustrates a preferred embodiment of the invention applied to a film sound recording system.

The output of a microphone l actuated by acoustic waves, or other source of signal cur rents, is amplified by the amplifiers 2 and 3 and the amplified signal currents supplied to the wires l and 5. A portion of the signal currents supplied to wires 4 and 5 flows in the primary winding of the transformer 6 and induces a current in the secondary winding of transformer 6 which flows through wire '7, movable elements 8 and 9 of the recording device, wire 10, resistor 11 and wire 12, thus causing the movable elements of the recording device to be displaced in accordance with the instantaneous variations of the signal currents.

Light from a source 13 is focused by the lens system 14 through aligned apertures 15 pierced in the pole faces of a permanent magnet 16 to form an image in or about the plane of the movable elements 8 and 9. The light emerging from aperture 15 is focused by the lens 1'7 on an aperture 18 pierced in an opaque plate 19. A light sensitive film 20 is traversed by the usual sprockets 21, 21 past the aperture 18. The magnetic field due to any electric current fiowing in the elements 8 and 9 will react with the permanent field of the magnet 16 to cause a displacement of the elements 8 and 9, thus varying the light impressed on the film 20. V

The recording device thus described is well known in the art and the present invention is not to be construed as limited to this particular form of light valve. Any other suitable recording device, such as the mirror galvanometer,

, Kerr cell, flashing lamp or other device, may be substituted for the device shown without departing from the scope of the invention.

A description of the adjustment of a light valve of .the type disclosed is given in an article The Principles of the Light Valve by T. E. Shea, W. l-Ierriott and W. R. Goehner published in the Journal of the Society of Motion Picture Engineers, volume XVIII, pages 597 to 730, June 1932.

The movable elements 8, 9 which are in the form of thin ribbon-like conductors, are mechanically supported, adjusted and tensioned to define a light transmitting slot of such height that the resultant time of exposure of the film 20 will produce an average exposure in the center of the selected range of the characteristic of the emulsion of the film 20. A unidirectional current flowing in the ribbons 8 and 9 will cause a change in the average height of the light transmitting slot and a corresponding change in average exposure of the film 20 and in the load carrying capacity of the recording device. Signal currents flowing in the ribbons 8 and 9 will cause the ribbons to oscillate about their average position as determined by the mechanical adjustment and the unidirectional current and cause a variation in the exposure of the film 20 in accordance with the instantaneous variations in the signal currents.

A portion of the signal currents flowing in the wires 4 and 5 is supplied to the potentiometer resistor 22. A controllable portion of the voltage developed across the resistor 22 may be applied through the adjustable slider 23 to the primary winding of a transformer 24. The output voltage developed in the secondary winding of the transformer 24 is amplified in a known mannor in a suitable amplifier, such as the thermionic triode 25. The amplified output of the triode 25 is supplied through a transformer 26 to a full wave rectifier 27, which may be composed of a pair of copper-copper-oxide couples. The rectified current flows from the rectifier 27, through inductor 28, resistor 29 and wire 30, through the secondary winding of transformer 26 back to rectifier 2'7. The current flowing in resistor 29 will cause the end of resistor 29 which is connected to inductor 28 to have a positive potential with respect to the other end. The inductor 28, combined with a capacitor 31 in parallel relation to resistor 29 smooths the rectified current, and causes the potential difference developed across the resistor 29 to vary with a desired characteristic of the signal currents, preferably with the envelope of the amplitude variations of the signal currents.

Current from a battery 32 flows through resistor 33 and high frequency inductor 34 to the output circuit of an oscillator 35, which may be a thermionic triode or other suitable device. The cathode of the oscillator 35 is ener ized by battery 36. Battery 37 impresses a suitable potential on the control electrode of oscillator 35 through the primary winding of transformer 38. The secondary winding of transformer 38 is connected through capacitor 39 with the anode of oscillator 35 and through a variable resistor 40 with the cathode of oscillator 35. A capacitor 41 is connected in parallel relation with the secondary winding of transformer 38. The oscillator 35 will produce an oscillating current, having a frequency largely determined by the effective inductance of the secondary winding of transformer 38 and the'capacitance of the capacitor 41 and an amplitude which may be varied by ad- J'usting the variable resistor 40. The frequency generated by the oscillator 35 should be outside the range of frequencies covered by the signal currents. In the recording of sound, a frequency of about 20,909 cycles per second will generally be found satisfactory.

High frequency current flows through capacitor 42 and the primary winding of transformer 43, and induces a corresponding voltage in the secondary winding of transformer 43. The outer ends of the secondary winding of transformer 43 are connected to the control electrodes of a pair of control devices 44 and 45, which may be threeelement thermionic triodes or other suitable electron discharge devices. The midpoint of the secondary winding of the transformer'es is connected through resistor 29 and polarizing bat tery 46 to the slider of a potentiometer 47 connected across the battery as exciting the oathodes of the control devices id and 45. A battery 49, connected to the cathodes of the control devices supplies current through inductor 50 and the two halves of the primary winding of transformer 51 to the anode circuits of he control devices 44 and 45.

The battery 46 and potentiometer 47 may be selected and adjusted so that, in the absence of signal currents, the high frequency waves substantially neutralize the negative polarizing potential applied to the control electrodes of the control devices 44 and i5, and may evencause the potential of the control electrodes to become slightly positive. The high frequency voltage applied to the-control electrodes is amplified by the control devices 4A and 45, and the amplified power flowing in the primary winding of transformer 51 induces a similar voltage in the secto be drawn together so ondary winding of transformerfil," This voltage causes a'current to flow through the full wave rectifier'5 2, inductor 53'to the slider of resistor 11. Here the currentjdivides, one" portion flowing through resistor 11,- wire12, and upper half of the secondary 'winding of transformer -6';' while the. other portion fiowsthrough wirel0, elements 9 and 8', wire 7, and lower half of the secondary winding of transformer 6. The magnetic effects of the two portions of the current flowing in the halves of the secondary Winding of transformer 6 are opposed and do not produce any magnetization of the core, thus avoiding saturation of the core and consequent distortion of the signal currents. The whole current then flows through wire 54 back to the midpoint of the secondary Winding of transformer 51.

The inductor 53, combined with the capacitor 55 form a low-pass filter that smooths the rectified output of transformer 51. The unidirectional current produced by rectifying and smoothing the high frequency current flows in the elements 9 and 8 and causes the elements that, in the absence of signal currents, the height of the light transmitting slot is less than'the normal height determined by the mechanical adjustment. The amount of reduction in the height of the slot depends on the magnitude of the unidirectional resistor 29 and causes the potential of the control 50 mitting slot may current, which is determined primarily by the constants of the oscillator circuit and the characteristics of the control devices 44 and 45 and may be varied by adjusting the resistor 40 and the potentiometer 4'7. The height of the transbe reduced to one-quarter or less of the normal height. A height of the light transmitting slot causes a corresponding reduction in the load carrying capacity of the light valve and also causes the mean exposure of the film 20 to be shifted nearer the region of under-exposure thus, in the absence of signal currents, producing a very light negative record and a correspondingly dark positive record.

As previously explained, when signal currents are supplied to the potentiometer 22, a voltage varying with the envelope of the amplitude variations of the signal currents is developed across electrodes of the control devices 44 and to become more positive with respect to the cathodes.

The control electrodes will then draw current from the cathodes and the impedance of the control'electrode-cathode circuit of the control devices is decreased. The control electrode-cath ode circuit of the control devices 44 and 45 is coupled through the transformer 43 to the input circuit of the oscillator 35 and is thus effectively in parallel with the input circuit of the oscillator 35. The decrease in the impedance of the con,- trol electrode-cathode circuit will thus cause a decrease in the amplitude of the oscillations of the oscillator 35, and, as this decrease is greater than the increased amplification of the control devices 44 and 45 due to the positive polarity of their control electrodes, the unidirectional current delivered by the transformer 51 decreases and the mechanical tension applied to the elements 8 and 9 acts to increase the height of thelight transmitting slot in accordance with the envelope of the amplitude variations of the signal currents, thus increasing the load carrying capacity of the light valve sufficiently to carry the increased load.

7 When the signal currents supplied to the amreduction in the vice will thus decrease rapidly with a rapid increase of the amplitude of the signal currents, that is, the operating time of the circuit is small. On the other hand, the restoring time is comparatively long. This asymmetric timing of the circuit has been found of value in certain types of sound recording.

What is claimed is:

1. In combination, a source of signal currents, a recording device actuated by the instantaneous variations in said signal currents, a recording medium, means for moving said medium past the recording point of said device, an oscillator having an input and an output circuit, a pair of control devices each having an input and an output circuit, said control devices being connected in push-pull relation, and the input cir- 29. The unidirectional I or biasing current supplied to the recording de-.

cuit of said control devices being effectively in parallel relation to the input circuit of said oscillator, an impedance element in the common portion of the input circuits of saidcontrol devices, means for'rectifying and filtering signal currents from said source, means for applying said filtered signal currents to said impedance element to cause the impedance of the input circuit of said control devices to vary inversely with said filtered signal currents, means for rectifying and filtering the output of said control devices and means for applying said filtered output to control the load carrying capacity of said recording device.

2. In combination, a source of signal currents, a recording device actuated by the instantaneous variations in said signal currents, a sensitive medium, means for moving said medium past the recording point of said device, a thermionic oscillator having a cathode, an anode and a con' trol electrode, a pair of thermionic control devices having cathodes, anodes and control electrodes, said devices being connected in push-pull relation, means for coupling the input circuit of said oscillator to the input circuit of said control devices, means for applying a negative potential to the control electrodes of said control devices substantially equal to the peak value of the positive waves from said oscillator, an impedance element in the common portion of the input circuit of said control devices, means for rectifying and filtering signal currents from said source, means for applying said filtered signal currents to said impedance element to cause the control electrodes of said control devices to become positively charged, means for rectifying and filtering the output of said control devices and means for applying said filtered output to vary the load carrying capacity of said recording device.

3. In combination, a source of signal currents, a recording device actuated by the instantaneous variations in said signal currents, a recording medium, means for moving said medium past pedance of the input circuit of said control device is varied inversely with said filtered signal currents, a rectifier and filter in the output of said control device, and means'for applying the filtered output of said control device to said recording device to vary the load carrying capacity of said recording device.

RAY R. SCOVILLE. 

